The invention relates to a method of stamping a bending edge in a layer of package material, such as cardboard or plastics, for use in the manufacture of a box or the like, formed with bends, by means of a stamping tool which is pressed down for forming the bending edge, as well as to a tool for performing the method.
In principle, package boxes are made by punching a cardboard workpiece in a punching tool. This workpiece is dimensioned such that by folding it can form the finished box.
To ensure a predetermined position of the bending edge, the workpiece is stamped to form a downwardly pressed bending edge in the bending line in a stamping tool. This is normally a working process which takes place at the same time as the punching of the cardboard workpiece.
Normally, this stamping takes place by means of a tool having a rib and cooperating lower part having a groove in which the downwardly pressed cardboard material and the rib may be received.
This method therefore requires a high degree of precision of the tool as well as of the machine, since the tool parts must be arranged and controlled very precisely in order to make the completely uniform slot at the right location in the cardboard workpiece.
Since, as mentioned, the stamping normally takes place at the same time as the actual punching of the workpiece, there are physical limits as to how far the stamping can proceed toward the end where the punching takes place. The reason is that the tool does not allow a procedure that permits full extension of the stamping tool since there is simply no room for the punching tool. Therefore, the stamped slot cannot extend in the full extent of the bending line but must stop a distance before the end edge at the cutting.
The only way of solving this will be to stamp in an independent working process, but since this is both an uneconomic and inaccurate way of production, this is normally not possible in practice.
The stamped slot itself, which consists of a downwardly pressed slot, forms a longitudinally extending bead on the lower side. When the workpiece is to be folded or bent, this normally takes place over the bead, which will thereby constitute an internally extending thickening in the bending line, while the outer side is formed by the bottom of the slot.
The actual folding or bending takes place over the downwardly pressed material part and will not be perfectly sharp in case of thick cardboard. This is caused partly by the width of the slot and partly by the different fiber direction that results in various tensile stresses in the edge area. In addition to this there are the problems involved by the circumstance that the slot does not extend right out to the end, so that in these areas the bending takes place entirely without stamping. All in all, this results in a disuniform external edge.
A further consideration is the tensile impact to which the cardboard is subjected by the bending, and which is so great that the cardboard has to be dimensioned to be able to accommodate this tensile stress. Therefore, the load is frequently the parameter according to which the entire cardboard must be dimensioned.
The specification of EP 0 189 909 A2 discloses an apparatus for stamping one or more slots in a layer by means of one or more balls or rolls which are advanced across the cardboard to form the slot. The advancement takes place by means of a programmable control of a carriage having a holder in which the stamping tool is mounted, said parts being movable in the x- and y-directions of the cardboard.
The stamping thus takes place in a progressing movement in the track of the bending line. This is a slow method which has been developed for the stamping of individual pieces. Therefore, this method cannot be used for a rational production of workpieces, since the apparatus can only stamp individual workpieces by a progressing downward pressing of the slot.